X-ray source for generating monochromatic x-rays

ABSTRACT

The present invention relates to an X-ray source comprising an electron source ( 1 ) for the emission of electrons (E), a target ( 4 ) for the emission of characteristic, substantially monochromatic X-rays (C) in response to the incidence of the electrons (E) and an outcoupling means ( 11 ) for outcoupling of the X-rays. To achieve characteristic, substantially monochromatic X-rays with a high power loadability electrons are incident on a metal foil ( 5 ) of a thickness of less than 10 μm and a base arrangement ( 7, 12 ) is arranged wherein the metal of said metal foil ( 5 ) has a high atomic number allowing the generation of X-rays (C) and the material substantially included in the base arrangement ( 7, 12 ) has a low atomic number not allowing the generation of X-rays (C). The outcoupling means are adapted for outcoupling only X-rays (C) on the side of the metal foil ( 5 ) on which the electrons (E) are incident and which is opposite to the side of the base arrangement ( 7, 12 ) since on this side almost no bremsstrahlung radiation is generated.

The present invention relates to an X-ray source comprising an electronsource for the emission of electrons, a target for the emission ofX-rays in response to the incidence of the electrons and an outcouplingmeans for outcoupling the X-rays. Further, the present invention relatesto a target for use in such an X-ray source.

An X-ray source of this kind based on the production of bremsstrahlungradiation in a turbulently-flowing liquid metal, also called LIMAX(Liquid Metal Anode X-ray source), is described in U.S. Pat. No.6,185,277. The electrons enter the flowing liquid via an electron windowwhich is a metal foil, for instance made of molybdenum or tungsten, or adiamond membrane. The electron window is sufficiently thin, inparticular a few μm, so that the electron beam loses only a smallportion of its initial energy in the window.

It is the object of the present invention to provide an X-ray source anda target for use in such an X-ray source which allows the generation ofsubstantially monochromatic X-rays, by which a significant dosereduction can be achieved and which permits a higher power loadabilitycompared to known X-ray sources.

This object is achieved according to the present invention by an X-raysource as claimed in claim 1 comprising:

an electron source for the emission of electrons,

a target for the emission of characteristic, substantially monochromaticX-rays in response to the incidence of the electrons, said targetcomprising a metal foil of a thickness of less than 10 μm and a basearrangement for carrying said metal foil, wherein the metal of saidmetal foil has a high atomic number allowing the generation of X-raysand the material substantially included in the base arrangement has alow atomic number not allowing the generation of X-rays, and

an outcoupling means for outcoupling the X-rays on the side of the metalfoil on which the electrons are incident and which is opposite to theside of the base arrangement.

A corresponding target for use in such an X-ray source is defined inclaim 14.

The present invention is based on the idea to provide a discrete lineX-ray source based on electron impact of a thin metal foil carried by abase arrangement. The basic idea is to discriminate against thebremsstrahlung radiation by observing the radiation emitted on the sideof the target on which the electrons are incident, i.e. the radiationwhich is essentially antiparallel to the initial electron beamdirection. The metal foil constituting the electron window is madesufficiently thin to preserve to a certain extent the angularcollimation of the electron beam incident on the foil. The foilthickness is less than the electron diffusion depth; hence, asignificant portion of the electron beam is deposited directly in thebase arrangement. Whether this is a good assumption in a particularsituation can only be ascertained by a simulation of the electron-photontransport, for instance a Monte-Carlo simulation. The power loadabilityof the proposed X-ray source is thus much greater than that of knownstationary anode X-ray sources.

Preferred embodiments of the invention are defined in the dependentclaims. While the invention generally works with a metal foil having athickness of less than 10 μm, the best results are obtained if the metalfoil has a thickness of less than 5 μm, preferably between 1 and 3 μm.

Furthermore, the metal foil is generally made of a metal which allowsthe generation of X-rays in response to the incidence of electrons. Thechoice of the material for the metal foil is dictated by the requiredphoton energy in the emitted X-ray beam. All metals with 20≦Z≦90, Zbeing the atomic number, are potential candidates, although metals withhigh mechanical strength, high melting point and ease of bondingtechnology with the base arrangement are favored. Preferred materialshave an atomic number between 40 and 80. Good candidate materials arefor instance tungsten, molybdenum or gold.

According to a preferred embodiment the base arrangement comprises acooling circuit arranged to allow a coolant to flow along the side ofsaid metal foil opposite to the side on which the electrons areincident, i.e. the metal foil is cooled by a flowing water beam dump. Toaid optimization of the design parameters of the known LIMAXarrangement, a simple approach has been taken to determine the maximumfocus temperature in dependence on such parameters of the liquid metalas the electron range, its diffusivity, flow velocity and degree ofturbulence. The diffusion model yields results which are in relativelygood agreement with those of a finite element program.

In the course of varying the input parameters to the above diffusionmodel the unexpected result was obtained that the thermal transport in awater-cooled arrangement leads to a factor of 10 increase in powerloadability at constant focus temperature relative to the best liquidmetal candidate. In quantitative terms, a focus of dimensions 1 mm×10 mmcould be loaded with an electron beam power of several tens of kWwithout exceeding the boiling point of water. This is exploited in thisproposed embodiment of the X-ray source to obtain the high powerloadability of the metal foil target by using a coolant having a lowatomic number avoiding the generation of X-rays therein.

While generally the coolant has a low atomic number preventing thegeneration of X-rays in response to the incidence of electrons, theatomic number is preferably less than 10. Such liquids include water aswell as oils based on hydrocarbon compounds. A high power loadability ofthe X-ray source has been, obtained by using water as a coolant.

To achieve a high flow velocity of the coolant in the area of the metalfoil, a cooling circuit in which the coolant is flowing comprises aconstriction in this area. Thus, a good cooling of the metal foil can beobtained and boiling of the coolant is prevented.

According to another preferred embodiment the target comprises a carriersupporting the metal foil on the side facing the coolant. Due to thevery low thickness of the metal foil, depending on the material of themetal foil, it can be necessary to support it in order to increasemechanical stability. In this case an appropriate carrier, for instancea thin diamond layer, can be provided.

For some medical applications of monochromatic X-rays in diagnosticradiology it is necessary to have a source of high radiance, andtherefore high pulse power, for a short exposure time (≦1 sec.). In apreferred embodiment of the present invention a rotating anode tubegeometry is used in which the base arrangement comprises a rotatablebase plate of a material having an atomic number of less than 10, inparticular in the range from 4 to 6. The base plate serves the functionsof supporting the thin metal foil and, when it is rapidly rotated, ofremoving by convection the electron energy deposited directly in thebase arrangement. The short term power loadability of this rotatinganode arrangement is at least a factor of ten greater than that of theembodiment comprising a cooling circuit, as the combination of the metalfoil and base plate can be operated at a much higher track speed and ata much higher temperature than the embodiment comprising the coolingcircuit. Therefore, this embodiment is a significant step towards arealistic monochromatic X-ray source for diagnostic radiology.

To avoid including bremsstrahlung radiation in the X-ray beam anoutcoupling means, such as an X-ray window transparent to X-rays, isprovided which generally only transmits X-rays propagating in thereflection direction of the metal foil, i.e. no X-rays in thetransmission direction are outcoupled. In a preferred embodiment theoutcoupling means only transmits X-rays propagating in a certain angularrange from the reflection direction as defined in claim 10. This ensuresthat almost only characteristic monochromatic X-rays are outcoupledsince bremsstithlung radiation almost completely propagates in thetransmission direction but neither in the reflection direction, nor insaid angular range.

According to another embodiment the outcoupling means is adapted tooutcouple X-rays in a direction substantially antiparallel to thedirection of incidence of said electrons, in particular in a directionat an angle in the range from 150° to 210° to the direction of incidenceof said electrons.

According to still another preferred embodiment the electrons aredirected onto the surface of the metal foil at an angle of substantially90°, i.e. perpendicular to the surface. In this direction the highestefficiency of producing X-rays can be ensured. However, to avoid theoutcoupled X-ray beam obstructed by the electron source, the electronsource is preferably located outside the X-ray beam, i.e. at an angledifferent from 90° to the surface of the metal foil. To ensure that theelectrons hit the metal foil at an angle of substantially 90°,appropriate means for directing the electron beam, for instanceappropriate deflection coils, are provided.

The present invention will now be explained in more detail withreference to the drawings in which

FIG. 1 shows the photon spectrum of a thick target of a known X-raytube,

FIG. 2 shows a polar plot of X-ray radiation from a thin W target,

FIG. 3 shows a first embodiment of an X-ray source according to thepresent invention comprising a cooling circuit,

FIG. 4 shows a photon spectrum of a thin target according to the presentinvention and

FIG. 5 shows a second embodiment of an X-ray source according to thepresent invention having a rotating anode tube geometry.

FIG. 1 shows the photon spectrum of a known X-ray tube having a targetwith a massive W anode in response to a 150 keV electron beam using a 2mm Al filter and a 10° anode angle. The ratio of photons in the almostdiscrete K lines to the total number of photons in the spectrum is ameasure for the monochromaticity M of the X-ray source. For the benefitof comparison with the X-ray source of the present invention the valueof M for the spectrum shown in FIG. 1 is about 10%. It is well knownthat electron diffusion makes a non-negligible contribution to thethermal transport in X-ray tube anodes. This contribution increases insolid-state, e. g. rotating anode X-ray tubes the shorter the time thatthe heat pulse has to diffuse through the target medium. The electrondiffusion component can dominate the thermal transport when the anodehas a relatively low conductivity. This is the case in a liquid anodetube when the anode consists of a coolant having a low atomic numberrather than a liquid metal having a high atomic number. Very high valuesof loadability, i.e. power loading per unit area of focus leading tounit temperature rise in the anode (loadability having a unit of W mm⁻²K⁻¹) can be achieved by this. A loadability for a liquid water anode of50 W mm⁻² K⁻¹ is feasible, and this is significantly higher than themaximum obtainable loadability with the known liquid metal anodes.

It is also established that the angular distribution of bremsstrahlungradiation is highly anisotropic for relativistic electron beams, with amarked preference for X-ray emission in the forwards direction. Thissituation is illustrated in FIG. 2 showing a polar plot ofbremsstrahlung intensity B for 128 keV electrons on free W atoms. Theatom is assumed to be at the center of the plot and the electron beampropagates vertically upwards as indicated by the arrow E. The intensityis proportional to the vector length from the center to the curve. Theangular distribution of characteristic radiation C is also shown. As canbe seen the angular distribution is isotropic, i.e. the intensity ofcharacteristic radiation is substantially equal in all directionsincluding the direction antiparallel to the direction of the electronbeam E. The cross sections for photon production are differential inphoton energy and emission angle.

These considerations together have led to the idea of a discrete lineX-ray source based on electron impact on a thin metal foil cooled by aflowing coolant beam dump, where the coolant is particularly water. Afirst embodiment of an X-ray source according to the present inventionis shown in FIG. 3. An electron source 1, for instance a cathode, emitsan electron beam E which under the influence of an external magneticfield generated by coils 2 rotates to enter the electron window 3 of thetarget 4 vertically. The electron window 3 comprises a thin metal foil 5of a material whose K lines are to be excited, supported if necessary bya thin carrier 6 of e. g. diamond.

The target 4 further comprises a cooling circuit 7 which can be a hollowtube in which a coolant 8 flows in the direction of the arrow 9. Inorder to increase the flow velocity of the coolant 8 in the area at theelectron window 3, in particular under the metal foil 5, the coolingcircuit 7 comprises a constriction 10 in this area, i.e. the crosssection of the cooling circuit 7 is reduced compared to the crosssection in other areas.

The thickness of the metal foil 5 is smaller than or equal to theelectron diffusion depth, which is the depth at which the energy lossper unit length projected on the incidence direction of the electronbeam E has its maximum value. It can be estimated from empiricalformulae, or rather derived from Monte-Carlo programs for the electrontransport. For 150 keV electrons incident on W foils its value isapproximately 4 μm. Selecting the thickness of the metal foil smallerthan or equal to the electron diffusion depth ensures that the electronvelocity vectors will not have had opportunity to become isotropicallydistributed in direction. In practice the thinness of the metal foilimplies that less than 20% of the electron energy is deposited in thefoil 5 or, correspondingly, that more than 80% of the energy isdeposited in the coolant 8.

The range of electrons of this energy is in tungsten approximately 20 μmfrom which it is evident that a significant proportion of the totalelectron energy will be deposited directly in the coolant. To a firstapproximation, the volume of coolant bombarded by electrons per secondis VRL, where V is the flow speed of the coolant 8 in the constriction10, L is the length of the electron focus perpendicular to the plane ofthe drawing of FIG. 3 and R is the electron range in water which ispreferably selected as a coolant. Hence the amount of energy this volumeof water can take up per second for temperature rise ΔT is VRL ΔT C_(p)where the last factor is the heat capacity of water (4.2 MJ m⁻³ K⁻¹). Ithas been assumed that the energy loss per unit length projected on theincidence direction of the electron beam E is constant over the electronrange. Inserting the values V=50 m s⁻¹, R=250 μm, L=10⁻² m, ΔT=25° leadsto a power of approximately 10 kW.

On the basis of the condition described above a foil thickness of lessthan 5 μm, preferably between 1 and 3 μm, for instance 2 μm, is assumed.Approximately 5% of the total power (about 1 kW) will be deposited inthe foil 5. A temperature rise of ΔT=50° is sufficient to remove thisheat load with a water flow speed given above.

As the assumed coolant has a low mean atomic number Z and the crosssection for production of bremsstrahlung is proportional to Z there willbe comparatively little X-ray production in the coolant.

The electrons penetrating through the foil 5 interact either bycollisional excitation to ionize the foil material or more occasionallythrough production of bremsstrahlung. The former involves the K shellelectrons if the incoming electron has sufficient energy. The excitedatom returns to its ground state by the emission of characteristicradiation e. g. with energy (K_(α1) line) of 57 keV. Characteristicradiation is emitted isotropically. The latter effect, bremsstrahlungradiation, is emitted almost completely in the direction oftransmission, i.e. in the downward direction in FIG. 3, while theintensity of bremsstrahlung emission in the direction of reflection,i.e. in the upward direction in FIG. 3, particularly in the directionperpendicular to the surface of the metal foil 5, is very low.

Hence, if the foil emission is observed in the direction of reflection,in particular over an angular range α of, preferably ±20° antiparallelto the direction of the electron beam, by use of appropriate outcouplingmeans 11, e.g. a window transparent to X-rays, it will be composed of abackground of low intensity bremsstrahlung from the coolant 8 on whichthe characteristic lines of the metal of the foil 5 are superimposed.This results in a quasi-monochromatic spectrum of high radiance C.Monochromatic radiation is useful in a number of areas of medical andscientific radiology including, but not limited to investigations withreduced patient dose, calibration of detectors and development of newdiagnostic modalities.

The mean energy loss by the electron beam E in the foil is approximatelygiven by the Thomson-Whiddington-law which is itself derived from theBethe-Bloch energy loss relationship. The Thomson-Whiddington-law is:E²=E₀ ²−xb ρ. E₀ is the initial electron energy and x is the foilthickness in the initial direction of the electron beam required toreduce the mean electron energy to E. The other symbols have theircustomer meanings.

The Thomson-Whiddington constant b has a value for tungsten of 8·10⁴keV²m²kg⁻¹ at 150 keV. This results in an energy loss per μm foilthickness of 5 keV for thicknesses which are small compared with theelectron range. The electron range is the value of foil thickness xrequired to reduce E to zero and is approximately 20 μm from thisequation.

A simulation result of the back-directed X-rays from the embodiment ofthe X-ray source shown in FIG. 3 having a 2 μm thick W foil irradiatedwith 150 keV electrons is represented in FIG. 4. The spectrum shows theradiation emitted in a cone of opening semiangle 15° in a directionantiparallel to the initial electron beam direction. Themonochromaticity parameter M defined above has a value of 0.45 for thisarrangement and can be improved further by optimizing the geometry, highvoltage and filtering.

FIG. 5 shows another embodiment of the present invention having arotating anode tube geometry in which the anode (i.e. the target) 4 isrotated. The design of this embodiment is taken from a dual-pole tube,i.e. the tube housing 13 is insulated from both cathode and anode HT viainsulators 14, as this design is most widespread in medical X-ray tubesfor short pulse exposures. The design is independent of the relativebias of the tube housing and anode, however, and can as easily berealized with a single pole X-ray tube.

Referring to FIG. 5, a high voltage electrode supplies the cathode 1with the necessary negative bias and current for the (e. g. thermionicemission) electron emitter. Through the action of an electrostatic orelectromagnetic beam deflection device (not shown), an electron beam Eis incident vertically upwards on the positively biased anode 4 in thecustomary way. The shape of the anode 4 and other details of the X-raytube design (insulators, cathode, bearings etc.) are well known to thosefamiliar with electron impact X-ray tube technology and will hence notbe discussed any further here.

The region of impact of the electron beam E at the anode 4 is shown inmore detail in the magnified inset to FIG. 5. The thin metal film 5 ofmaterial (e. g. W, Mo etc.) whose K characteristic radiation is to beexcited is deposited on an anode base material 12. The metal film 5 hasa thickness T, where T≦D, D being the electron diffusion depth.

Opposite to the anode 4 in the tube housing 13 is the exit window 11 ofthe X-ray tube which is arranged to select only that radiation from theanode 4 which is emitted antiparallel (160°≦θ≦180°) to the electron beamdirection of incidence. As described for the first embodiment, thisselection, together with the condition on the film thickness T, ensuresthat the X-ray beam consists predominantly of the quasi-monochromatic Kcharacteristic lines of the metal film 5.

The material of anode base plate 12 should have low Z, to absorbelectron energy without producing bremsstrahlung X-rays. Materials witha high melting point, high thermal conductivity and a high thermalcapacity are advantageous. Two obvious candidates for the anode baseplate 12 are beryllium (Be) and graphite (C). The latter is in any casewidely used in X-ray tubes which have a high heat storage capacity onaccount of their good thermal conductivity (150 W m⁻¹ K⁻¹) and highspecific heat of 700 J kg⁻¹ K⁻¹.

The combination W film on a graphite has been investigated and isapparently stable to temperatures higher than 1000° C. Metal films canalso be deposited (e. g. by electroplating) on Be although there seemsto be a problem with diffusion into the Be at high temperatures. Aplatinum (Pt) buffer layer of 0.1 μm thickness between the metal film 5and the anode base plate 12 may be necessary.

The power loadabihty of the arrangement of FIG. 5 is analogous to thatperformed above in connection with the description of FIG. 3. when thethermophysical parameters of the coolant are replaced by those of theanode base material. Use of the values V=50 m s⁻¹, R=100 μm, L=10⁻² m,ΔT=1000° C., with C_(p)=700 J kg⁻¹ K⁻¹ and ρ=2500 kg m⁻³ (graphite)leads to an instantaneous power on a cold anode of ˜100 kW for a 1 mm²focus. The loadability will obviously decrease as the graphite basewarms up. The extent to which this occurs depends on design details ofthe graphite base e. g. its thickness (parallel to the axis of rotationof the anode) and the diameter of the anode.

1. An X-ray source comprising: an electron source for the emission ofelectrons, a target for the emission of characteristic, substantiallymonochromatic X-rays in response to the incidence of the electrons, saidtarget comprising a metal foil of a thickness of less than 10 μm and abase arrangement for carrying said metal foil, wherein the metal of saidmetal foil has a high atomic number allowing the generation of X-raysand the material substantially included in the base arrangement has alow atomic number not allowing the generation of X-rays, and anoutcoupling means for outcoupling the X-rays on the side of the metalfoil on which the electrons are incident and which is opposite to theside of the base arrangement.
 2. The X-ray source as claimed in claim 1,wherein said base arrangement comprises a rotatable base plate of amaterial having an atomic number of less than 10, in particular in therange from 4 to
 6. 3. The X-ray source as claimed in claim 1, whereinsaid base arrangement comprises a cooling circuit arranged to allow acoolant to flow along the side of said metal foil opposite to the sideon which the electrons are incident.
 4. The X-ray source as claimed inclaim 3, wherein the coolant has a mean atomic number of less than 10.5. The X-ray source as claimed in claim 3, wherein the coolant is water.6. The X-ray source as claimed in claim 3, wherein said cooling circuitcomprises a constriction in the area of the metal foil.
 7. The X-raysource as claimed in claim 3, wherein said target further comprises acarrier of low atomic number material, in particular having a meanatomic number of less than 10, supporting the metal foil on the sidefacing the coolant.
 8. The X-ray source as claimed in claim 1, whereinthe metal foil has a thickness of less than 5 μm, preferably between 1and 3 μm.
 9. The X-ray source as claimed in claim 1, wherein the metalof said metal foil has an atomic number between 40 and
 80. 10. The X-raysource as claimed in claim 1, wherein said outcoupling means is adaptedto outcouple X-rays at angles of an angular range from substantially 45°to 135°, in particular 70° to 110°, to the surface of the metal foil.11. The X-ray source as claimed in claim 1, wherein said outcouplingmeans is adapted to outcouple X-rays in a direction substantiallyantiparallel to the direction of incidence of said electrons, inparticular in a direction at an angle in the range from 150° to 210° tothe direction of incidence of said electrons.
 12. The X-ray source asclaimed in claim 1, wherein said electrons are directed onto the surfaceof said metal foil at a substantially 90° angle.
 13. The X-ray source asclaimed in claim 1, wherein said electron source is located outside theX-ray beam to be outcoupled, said X-ray source further comprising meansfor directing the electron beam onto the metal foil.
 14. A target foruse in an X-ray source for the generation of characteristic,substantially monochromatic X-rays in response to the incidence ofelectrons, said target comprising a metal foil of a thickness of lessthan 10 μm and a base arrangement for carrying said metal foil, whereinthe metal of said metal foil has a high atomic number allowing thegeneration of X-rays and the material substantially included in the basearrangement has a low atomic number not allowing the generation ofX-rays.
 15. An x-ray source comprising: an electron source for theemission of electrons, and a target for the emission of substantiallymonochromatic x-rays in response to the incidence of the electrons, saidtarget comprising a metal foil and base arrangement, said metal foilallowing the generation of x-rays and the base member not allowing thegeneration of x-rays.
 16. The x-ray source as claimed in claim 15,wherein said base arrangement comprises a cooling circuit to allow acoolant to flow along the side of said metal foil opposite to the sideon which the electrons are incident.
 17. The x-ray source as claimed inclaim 16, wherein the coolant is water.
 18. The x-ray source as claimedin claim 16, wherein said cooling circuit comprises a constrictionproximate the metal foil.